The invention relates to fuel cell desiccants.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a polymer exchange membrane, also more generally referred to as a proton exchange membrane, both references commonly abbreviated xe2x80x9cPEMxe2x80x9d) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons formed at the anode side of the membrane electrode assembly represent oxidation of the anode gas during the fuel cell reaction. Electron consumption at the cathode side of the membrane electrode assembly represents reduction of the cathode gas during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
xc2xdO2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
As shown in equation 1, the hydrogen forms protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
PEM fuel cell membranes have been made from perfluorosulfonic acid polymers such as NAFION(trademark). It can be advantageous to hydrate such membranes to maintain performance. As a result, it is common for the maximum operating temperature for such membranes to be less than 100xc2x0 C. (e.g., about 80xc2x0 C.) at atmospheric pressure. At such conditions, another problem may arise in fuel cell systems where the hydrogen fuel stream is derived from a processed hydrocarbon such as natural gas or methanol, in that carbon monoxide may be present as a by-product of the fuel conversion process, and carbon monoxide can poison most fuel cell catalysts (e.g., platinum based) at temperatures of about 100xc2x0 C. or lower. Therefore, it can be desirable to provide a higher temperature, CO tolerant fuel cell system.
Acid-doped polybenzimidazole (PBI) membranes have been developed that may be operated at temperatures above 100xc2x0 C., for example 150-200xc2x0 C. Such membranes are disclosed, for example, in U.S. Pat. Nos. 5,525,436, 5,599,639, and 5,945,233, which are hereby incorporated by reference in their entirety. In such membranes, a PBI film is typically synthesized and then doped with a strong acid such as phosphoric acid or sulfuric acid to produce a polymer with the acid anion bound to the protonated PBI.
This invention relates in part to the discovery that high temperature acid-doped fuel cell membranes can be damaged if water is allowed to condense onto such membranes as they are cooled. For example, while the relative humidity of fuel cell reactants in a xe2x80x9chigh temperaturexe2x80x9d PEM fuel cell operating at temperatures above 100xc2x0 C. (e.g., 150-200xc2x0 C.) may be low, the dew point of such a mixture may be such that liquid water will condense onto the membranes in the fuel cell stack when the stack is cooled from its operating temperature.
As previously discussed, some solid electrolytes are formed of PBI that is doped with phosphoric acid. The phosphoric acid is added to the PBI to enhance the proton conducting properties of the electrolyte. In general, the interaction of water vapor with the PBI electrolyte during operation of the fuel cell or system does not leach the phosphoric acid from the PBI electrolyte. However, when exposed to liquid water (e.g., condensed water vapor formed upon cooling of a fuel cell or fuel cell system after shut down), the liquid water can leach the phosphoric acid from the electrolyte, rendering the electrolyte less useful (e.g., by decreasing the ability of the PBI electrolyte to conduct protons).
Under certain embodiments of the present invention, a desiccant system is provided to prevent water condensation in high temperature PEM fuel cells. A desiccant can be placed in the fuel cell or system so that, after shut down, water (e.g., condensed water vapor) can sorb to the desiccant as the fuel cell or system cools. Furthermore, because fuel cells and fuel cell systems typically operate at temperatures greater than about 100xc2x0 C., water sorbed on the desiccant can be desorbed during operation of the fuel cell or system, allowing the desiccant to be regenerated for re-use in the fuel cell or system. Thus, a fuel cell or a fuel cell system containing a water-sensitive solid electrolyte and a desiccant can demonstrate a relatively long useful life and/or relatively good performance. The fuel cell or the fuel cell system can be simple and/or inexpensive to make.
Generally, the desiccant has a higher water sorption capacity at room temperature than at the operating temperature of a fuel cell or fuel cell system (e.g., greater than about 100xc2x0 C.). The desiccant can be formed of, for example, molecular sieves, silica gels and/or clays. In some embodiments, the desiccant is formed of granules (e.g., granules in a water-permeable container). In certain embodiments, the desiccant is in the form of a compressed block. The desiccant can be in the form of a coating (e.g., a coating on a portion of the fuel cell or fuel cell system that is exposed to water vapor, such as a manifold or a channel in a flow field plate). More than one desiccant can be used.
In one aspect, the invention features a method of managing water in a fuel cell having a solid PBI electrolyte. The method includes operating the fuel cell at a temperature above about 100xc2x0 C., shutting down the fuel cell so that the fuel cell cools, and contacting a reactant gas with a desiccant disposed within the fuel cell. The desiccant has a higher water sorption capacity at room temperature than at 100xc2x0 C. so that water present in the reactant gas sorbs to the desiccant as the fuel cell cools.
The method can further include, after shutting down the fuel cell (e.g., cooling to less than about 100xc2x0 C.), operating the fuel cell at a temperature above about 100xc2x0 C. so that water sorbed to the desiccant desorbs from the desiccant. Optionally, after shutting down the fuel cell, the fuel cell can be valved off so that gases cannot flow through the fuel cell. In some embodiments, the reactant gas may contact the desiccant as the reactant gas flows through the fuel cell in a reactant gas stream. In still other embodiments, reactants may be circulated across the desiccant as the fuel cell or system cools.
In another aspect, the invention features a fuel cell stack that includes at least two fuel cells, a manifold and a desiccant disposed in the manifold. One fuel cell includes two flow field plates and a solid PBI electrolyte between the flow field plates. The other fuel cell includes two flow field plates and a solid electrolyte therebetween. Each flow field plate has an inlet in fluid communication with its respective outlet. The inlet and outlet of one of the flow field plates in one fuel cell are in fluid communication with the inlet and outlet, respectively, of one of the flow field plates in the other fuel cell. Likewise, the inlets and outlets, respectively, of the other two flow field plates in the fuel cell stack are in fluid communication. The manifold is in fluid communication with the inlet of one of the flow field plates from each fuel cell. The desiccant is capable of sorbing water vapor present in the fuel cell stack to reduce the amount of water that sorbs to the solid PBI electrolyte.
The fuel cell stack can include one or more additional manifolds in fluid communication with the inlet and/or outlet of one or more of the flow field plates. One or more additional desiccant(s), formed of the same or different material(s), can be disposed in the additional manifold(s).
In a further aspect, the invention features a fuel cell system that includes a fuel cell having two flow field plates, a solid PBI electrolyte disposed between the flow field plates, an inlet line, and outlet line, two valves and a desiccant. The inlet and outlet lines are in fluid communication with the inlet and outlet, respectively, of one of the flow field plates. One valve is disposed along the inlet line and is capable of restricting gas flow through the inlet line. The other valve is disposed along the outlet line and is capable of restricting gas flow through the outlet line. The desiccant is disposed inside the fuel cell system between the valves.
In some embodiments, the desiccant is in the inlet and/or outlet line (e.g., the cathode gas inlet line, the cathode gas outlet line, the anode gas inlet line and/or the anode gas outlet line).
In certain embodiments, the desiccant is coated on a portion of the fuel cell system (e.g., an inlet line, an outlet line, a manifold, and/or one or more channels of one or more flow field plates).
In another aspect, the invention features a fuel cell that includes two flow field plates, a solid PBI electrolyte between the flow field plates, and a desiccant on a portion of one of the flow field plates. The desiccant is capable of sorbing water vapor present in the fuel cell to reduce the amount of water that sorbs to the solid PBI electrolyte.
The desiccant can be coated on one or both of the flow field plates (e.g., on one or more channels of the flow field plate(s)).
Other features, objects, and advantages of the invention will be apparent from the description, drawings and claims.